Dual-Functional Lithiophilic/Sulfiphilic Binary-Metal Selenide Quantum Dots Toward High-Performance Li–S Full Batteries

Highlights A bi-service host with lithiophilic/sulfiphilic Fe2CoSe4 quantum dots embedded in three-dimensional ordered nitrogen-doped carbon skeleton is elaborately developed for both the sulfur cathode and Li anode synchronously. The highly dispersed Fe2CoSe4 quantum dots can not only act as a redox accelerator to promote the bidirectional conversion of LiPSs but also regulate the uniform Li plating/stripping to mitigate the growth of Li dendrite. The assembled Li-S full batteries achieve excellent long-term cyclability and a remarkable areal capacity of 8.41 mAh cm2 at high sulfur loading of 8.50 mg cm2, and the pouch full battery also displays high capacity and cycling-stability at lean electrolyte condition. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01037-1.

For the symmetric cell assembly and measurements, the working electrode was prepared by mixing host matrices materials and PVDF at a weight ratio of 9:1. Symmetric cells were assembled by using two identical electrodes with a Celgard 2400 membrane as the separator, and 60 µL of Li2S6 electrolyte (containing 1 mol L -1 LiTFSI, 2 wt% LiNO3, and 0.2 M Li2S6 in DOL/DME solution with a volume ratio of 1:1) as electrolyte. The CV curves of symmetric cells were performed within the voltage range of -1.0-1.0 V (vs. Li + /Li). EIS was tested by Autolab electrochemical workstation (NOVA 1.9) with a frequency ranging from 0.01 Hz to 100 kHz.
For the Li2S nucleation and decomposition measurement, the cells were assembled by the above active electrodes as working electrodes and Li foils as counter electrodes. 25 µL of 0.5 M Li2S8 and 1.0 M LiTFSI dissolved in a tetraglyme solution were used as the catholyte, while 25 µL of 1.0 M LiTFSI dissolved in a tetraglyme solution as the anolyte. For the nucleation test, the cells were galvanostatically discharged to 2.06 V at 0.112 mA and then kept potentiostatically at 2.05 V until the current dropped below 10 -5 A. For the decomposition process, the cells were galvanostatically discharged to 1.7 V at a constant current of 0.10 mA, then continue galvanostatically discharged to 1.7 V at 0.01 mA, and final potentiostatically charged at 2.40 V for 20,000 s.
For the LSV measurements, a three-electrode configuration was fabricated using glassy carbon coated with active material as the working electrode, Ag/AgCl electrode as the reference electrode, platinum sheet as the counter electrode, and 0.1 mol L -1 Li2S/methanol solution as the electrolyte. The LSV tests were conducted using the CHI660D electrochemical workstation (Shanghai Chenhua Device Company, China) from -0.8 V to -0.1V at a scan rate of 5 mV s -1 .

S1.3 Electrochemical Measurements
For the sulfur cathode test, the resulting cathode electrodes, Li anode, and Celgard 2400 separator were employed to assemble the CR2032 coin-type Li-S batteries in an argonfilled glove box (<1 ppm of O2 and H2O). The electrolyte was lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (1 M) in a mixed solvent of 1,2dimethoxyethane and 1,3-dioxolane (1:1, v/v) with 2 wt% of LiNO3 additive, and the E/S ratio of half-cell tests was maintained at about 20 µL mg -1 . Cycling and rate performance tests in the cutoff potential of 1.8-2.7 V (vs Li + /Li) were performed on a Neware battery test system (CT-4008, 5 V-10 mA, and 5 V-50 mA). CV curves were recorded in the voltage range from 1.7-2.8 V (vs Li + /Li).
For the synthesis of Li metal hybrid anodes, the 3DIO FCSe-QDs@NC (or FCSe-QDs@NC and 3DIO NC) was firstly mixed with PVDF in NMP solvent with a mass ratio of 9:1, coated onto the Cu foils, and cut into discs with the mass loading of about 1.5 mg cm -2 . The Li/3DIO FCSe-QDs@NC, Li/FCSe-QDs@NC, Li/3DIO NC, and Li@Cu anodes were prepared by electrochemical deposition. The deposition process was performed using CR2032 coin cells with Li foils as the counter electrode and the aforementioned Li-S battery electrolyte as the electrolyte. Prior to the test, the cell was cycled at 50 µA between 0.01 and 3.0 V (vs Li + /Li) for 5 cycles to form a stable SEI film. For Li symmetric battery configurations, both working and reference electrodes are the Li-deposited electrodes. Then, 15 mAh cm −2 of Li was deposited on the host through galvanostatic discharging. The cycling performances were recorded under a variety of current rates and capacities. The full cells were constructed by the obtained sulfur cathodes and the corresponding pre-deposited Li anodes with a N/P ratio of lower than 5 for routine tests (a Li deposition capacity of 12 mAh cm -2 ).

S1.4 Computational Simulation
The density functional theory (DFT) calculations were operated in the Vienna Ab-initio Simulation Package (VASP) [S1, S2]. The projector augmented waves (PAW) in the Perdew-Burke-Ernzerhof (PBE) form were chosen as the pseudopotentials [S3]. The Brillouin zone of the supercell was sampled by a 2 × 2 × 1 uniform k-point mesh. The energy cutoff of the plane base sets was 500 eV. All the atoms in the structures were relaxed until the residual forces were less than 0.01 eV Å -1 and the total energy difference was less than 10 -5 eV. The binding energy ( ) was defined as the energy difference of adsorbed model ( / 2 ) and the summation of pure Li2Sn ( 2 , n = 1, 2, 4, 6, 8) molecule and the surface energy ( ) according to = , where more negative values indicated stronger binding interaction. The transition state of Li2S decomposition on the surface was located by the nudged elastic band (NEB) method. All the calculation models adopted in this work were conducted with the ALKEMIE platform [S4]. Charge density difference was obtained from the charge difference between the substrate and the adsorbent.
Theoretical expressions of the current-time transients of four classic electrochemical deposition models were presented as follows: The reaction can be written as:

Supplementary Figures and Tables
6 2 4 + 37 2 → 6 2 3 + 2 3 4 + 24Se 2 Therefore, the weight ratio of Fe2CoSe4 in 3DIO FCSe-QDs@NC is calculated to be about 81 wt%    The baseline voltage and current density are defined as the value before the redox peak, where the variation in current density is the smallest, namely dI/dV = 0. Baseline voltages are denoted in Cambridge blue for cathodic peak Ⅰ, Ⅱ, and in black for anodic peak Ⅲ, respectively. The CV curves and corresponding onset current density is 10 μA cm -2 beyond the corresponding baseline current density (more specifically, 10 μA cm -2 more negative than baseline current density for the cathodic peaks or 10 μA cm -2 more positive than baseline current density for anodic peaks). As shown in the inset, the baseline voltages are exhibited, and the colored region indicates the gap in current density (10 μA cm -2 ) [S5]. Wherein Ip is the peak current density, n is the number of electrons during reactions, A is the electrode area, Li + is the Li + ion diffusion coefficient, C is the concentration of Li + ion in the electrolyte, and v is the scanning rate. The higher the slope, the stronger the ion diffusion ability [S7, S8]. Note: On the 3DIO FCSe-QDs@NC electrode surface, the Li2S exhibits 3D granularmorphology and is evenly deposited without obvious aggregation of large particles, which is mainly driven by the abundant catalytic sites in the 3D porous carbon skeleton. Moreover, the smooth LiPSs diffusion and efficient charge transfer profited from the conductive carbon skeleton contribute to the rapid Li2S nucleation and uniform deposition on the Li2S/host/electrolyte three-phase interface [S9]. In stark contrast, there are many sheet-like self-assembled Li2S agglomerates deposited on the bulk FCSe-QDs@NC electrode surface, on account of the limited amount of active sites exposed on bulk surfaces and rapidly depleted during the deposition process, which has been proved to be universal in ether-based electrolytes (Fig. S17) [S10]. For the catalyst-free, the deposited Li2S completely exhibits a 2D sheet-like morphology with a rough and uneven surface. And such sheet-like structures would lessen the three-phase interface mediated for the electron/ion transfer and further impedes the subsequent growth of Li2S (Fig. S18)     The in-situ formed Co and Fe phase works as the preferred nucleation sites for the subsequent Li deposition. While the formed Li2Se phase possesses high ionic conductivity, which is favorable for fast Li-ion diffusion. Such mixed conductive phases have been proven to effectively regulate the nucleation and deposition of Li metal [S13-S15]